In wireless (cellular) communications, an uplink signal at a base station antenna usually fluctuates as a result of fading caused by multiple reflections at buildings and obstacles. To reduce this fading effect, prior art base stations may have an additional antenna for the same sector to provide space diversity. This type of antenna system, however, is bulky and is generally considered to be aesthetically unpleasing. Another known way to reduce fading is through polarization diversity, i.e. reception of signals on two orthogonal polarizations (usually slant polarizations of +/−45°). Polarization diversity allows a decrease in the number of antennas by two times in comparison with space diversity. However, the base station still needs at least three antennas for a three-sector operation. In an urban environment, polarization diversity provides signal quality similar to space diversity. At the same time, in urban areas, the visual impact of a base station antenna has become a big concern, especially in historical or fine art architecture districts.
As is well known in the art, three polarization diversity antenna arrays can be combined in one cylindrical radome to decrease their visual impact and reduce the number of antennas for a base station to just one. Each vertical array for a 120° sector is constructed using slant 45° crossed dipoles located above a ground plane. If the diameter of this three-sector antenna is small enough, it can be used as part of a light pole, flagpole, or even as an element of church cross, so that the antenna can be invisible in the environment. Hence, it is very important to decrease the diameter of the antenna. At the same time, it is very important for an antenna to have good mechanical strength such that it can be used as an element of some structures.
Notably, prior-art three-sector antennas do not find wide field of application. One reason is their large diameters, as was discussed above. Another main reason is the need for the sector optimization.
One main method to optimize the coverage area of an antenna beam is tilting the beam downward (mechanically or electrically) from the horizontal axis in the vertical plane. More down tilt achieves a smaller cell size. In the case of a three-sector antenna, each of the 3 antenna arrays often need to have different beam tilts to suppress the interference with adjoining cells, and to provide the cell size optimization because conditions are usually quite different in different directions. Conventionally, mechanical down tilt does not work well for a three-sector antenna. To make a three-sector antenna more universal, it needs to have electrical variable down tilt for each of three sectors.
To provide a variable down tilt, an antenna may have adjustable phase shifters incorporated with its feed lines. A one-sector antenna variable phase shifter may consist of a dielectric block on a meander line moving orthogonal to its axis. This type of phase shifter has significant lateral dimensions, and cannot be used in a three-sector array without increasing of it's diameter.
Another big issue for every base station antenna is intermodulation (IM). The main method to minimize IM is to avoid metal-to-metal contacts.
Another problem with prior dual polarized dipole arrays with variable tilt is beam squint in the horizontal plane (up to 12° with 10° tilt).
As well known in the art, the mutual coupling between crossed dipoles influences correlation of the two orthogonal polarized signals, and can disturb the effect of polarization diversity. When three antennas are combined together, the effect of mutual coupling becomes even worse. To provide polarization diversity, dual polarized base station antennas have to meet a certain port-to-port isolation specification (typically more than 30 dB), and a certain level of cross-polarization (the co-pol to cross-pol ratio must be more than 10 dB in all 120° sectors).
Another challenge with three-sector antennas is back radiation. Back radiation is characterized by front-to-back (F/B) ratio, which usually needs to be more than 25 dB. Wider antenna ground plane gives better F/B. With narrower ground plane F/B can degrade.
It is one principal object of the present invention to provide a dual polarized antenna array with a compact package.
It is a further object of the invention to provide a dual polarized antenna array with a variable beam tilt.
It is another object of the invention to provide an antenna capable to meet at least 30 dB port-to-port isolation.
It is another object of the invention to provide an antenna array capable to meet at least a 10 dB co-pol to cross-pol ratio in a 120 degree horizontal sector.
It is another object of the invention to provide an antenna array having a 65–85° horizontal beamwidth.
It is another object of the invention to provide an antenna array with a front-to-back ratio of more than 25 dB.
It is a further object of the invention to provide a dual polarized antenna with a high gain.
It is further object of the invention to provide a dual polarized three-sector antenna having a variable beam tilt with small (less the wavelength) diameter of radome.
It is another object of the invention to provide an antenna array with minimized intermodulation.
It is further object of the invention to provide a inexpensive antenna.